Dielectric material and method of producing it



Patented June 16, 1936 UNITED, STATES DIELECTRIC MATERIAL METHOD OFPRODUCING IT Leon McCulloch, Pittsburgh, Pa., assignor to WestinghouseElectric & Manufactunng Company, East Pittsburgh, Pa., a corporation ofPennsylvania .No Drawing. Application December 28,1933,

Serial No. 704,368

8 Claims. (Cl. 154-2.6)

My invention relates to improving the dielectric quality of textilematerials and particularly to dielectric materials, such as those havingan asbestos base, and to a process for making the same.

It is well known that asbestos and similar materials have ratherpoordielectric characteristics because of the presence of conductingmineral impurities in the commercial grades of such material, and alsobecause of the presence of moisture which makes thematerial slightlyconductive. Numerous attempts have been made to remove the iron oxideswhich are the principal mineral impurities found in asbestos, but, sofar as is known, none of these has proved successful.

Attempts have also been made to overcome the conductance of asbestos dueto the presence of moisture therein by making the material vaporiree asa result of exposure to heat or evacuation,

and then coating the material with a non-hygroscopic substance.

I have previously developed a method of increasing the dielectric valueof asbestos while heating the fibres to a charring temperature with oilsor other organic matter.

The low resistance of asbestos tape, which is used as an insulatingmaterial on the windings of high voltage rotating machinery, has beenresponsible for corona losses in the end windings of the machines, andsuch insulation has, in certain cases, failed in service as a result ofsparking due to surface leakage through the asbestos wrapper surroundingthe winding. It is, therefore, desirable to find a satisfactory tape ortreatment for the tape that will improve its natural dielectricqualities.

In order to make the asbestos fibres less hygroscopic, I have triedtreating them by washing with a number of difierent substances, such asoxalic acid or phthalic acid, with practically no improvement in thedielectric qualities of the tape. I have also attempted to remove thewaterloluble impurities from the tape by treating in a Soxhlet apparatuswith water, but without any appreciable improvement in the dielectricvalue.

Heat treatments were also tried, baking the asbestos in air at varyingtemperatures. At temperatures up to about 435 0., no improvement in thedielectric strength occurred. At about 500 0., there was a definiteincrease in resistance value. At this temperature, however, there was apartial weakening of the fibres caused by dehydration.

Such tapes have also been treated by baking treatment, the cottoncomprising a part of the tape structure decomposed and left the fibrescoated with organic matter making a tape of highelectrical resistance ofabout one-half the tensile strength of the untreated tape.

A number of specimens of different kinds of asbestos tape were testedunder a large number of different treatments with an attempt to increasethe dielectric value thereof, and tests madeof the dielectric strengthby placing one inch length of tape between two pairs of small brassplates clamped together by spring test clamps. The resistance tests weremade by applying 550 volts direct current to the one inch length oftape, the current being read by a reflecting galvanometer. Breakdown orfiashover tests were made with specimens of tape clamped in the samemanner with 60 cycle voltages increased by steps of 2 kilovolts appliedfor one minute each or until fiashover occurred.

I found by these tests that the resistance of 20 asbestos tapes could bevery considerably increased by treating the tapes with a solution ofboric acid. Such tapes treated with a solution of from 2% to 10% ofboric acid give very satisfactory results from a dielectric standpoint.The 25 tapes treated in the boric acid solutions containing the higheramounts of boric acid in solution tend to become somewhat stiff for easyapplication where. the tape is to be applied over very small conductingparts. Specimens of tape dipped in a boric acid solution of about 5%appeared somewhat better dielectrically than specimens treated with aweaker or stronger solution of the boric acid. The tape treated in thissolution appeared to be particularly free from smoking or charring athigh voltages, and to have a particularly high fiashover voltage and toalso retain a high dielectric value after several flashovers.

The material was prepared by soaking the untreated tape from twelve toeighteen hours in the boric acid solution, draining the tape for aboutone hour, and then drying it by heating it in an oven at a temperatureof about 100 C. for about eighteen hours.

After the tape was treated by soaking in the boric acid solution asdescribed above, and draining, the increase in weight due to theabsorbed solution would be 60% or 62% of the weight of the untreatedmaterial, or, if a 5% solution of boric acid were used in the treatmentabout 3.5% by weight of boric acid.

I have also treated tapes by dipping them in a t 250 C. out of contactwith air. During this boric acid solution in the rolled condition, and

dried as described above, with satisfactory results. The dipping may bedone by passing the tape continuously through the solution. The tapeshould remain in the solution long enough to thoroughly wet or saturatethe fabric.

While a solution of boric acid in water produces satisfactory results,other solutions, such as an alcoholic solution of boric acid may beused,

if desired.

It is to be understood that the description of particular embodiments ofmy invention is to be taken as illustrative of the principle and modethereof, and that modifications may be made within the spirit of theinvention without departing from the scope of the appended claims.

I claim as my invention:

1. A method of treating asbestos tape to improve its dielectricproperties comprising soaking the tape in a five percent solution ofboric acid, for from ten to eighteen hours, draining, and drying in anoven at a temperature of about C.

2. A method of treating textiles, such as asbestos, to improve itsdielectric properties comprising soaking it in a solution of boric acidof a strength from two to ten percent, draining, and drying at atemperature of about 100 C.

3. A method of treating textiles, such as asbestos, to improve itsdielectric properties comprising soaking it in a solution of boric acidfor a sufiicient time for the material to take up from 3 to 4 percent byweight of the untreated material of boric acid, draining, and dried forabout eighteen hours at a temperature of about 100 C.

4. A dielectric tape material comprising a tape of textile material thathas been soaked in a solution of boric acid for 1 to 18 hours, or tillsaturated, drained, and dried by heating to about 100 C. for abouteighteen hours.

, 5. A dielectric tape comprising textile material that has been soakedin a solution of boric acid fora sufficient time to take up from 3 to 4percent boric acid by weight, and from which the moisture added duringthe soaking process has been substantially removed.

6. A dielectric material comprising asbestos or like textiles that hasbeen soaked in a 2 to 10 percent solution of boric acid for 1 to 18hours, or till saturated and from which the added moisture has beensubstantially removed.

'7. A dielectric material comprising asbestos or like textiles that hasbeen soaked in a 2 to 10 percent solution of boric acid for 1 to 18hours, or till saturated, drained, and dried at an elevated temperaturefor about eighteen hours.

8. A method of treating asbestos, or like material, to improve itsdielectric properties, which consists in soaking it in a 2 to 10 percentsolution of boric acid for a sufiicient time to take up 3 to 4 percentof its weight of boric acid, and in subsequently removing the moistureadded during the soaking process.

LEON MCCULLOCH.

